The present invention relates to a method of preparing a mixture of cation exchange resins and anion exchange resins in a size range of about 10 microns (less than 400 mesh) to about 250 microns (60 mesh) from resins larger than about 60 mesh, for use in precoating a filter. The 10 to 250 micron size range of ion exchange resins is particularly suited to reduce dissolved and undissolved impurities in solution from a level of about 50 parts per billion to about 10 parts per billion or less.
In the prior art, methods and apparatus, such as a filter, using a precoat of ion exchange resin particles smaller than 250 microns (60 mesh) are known, as set forth in U.S. Pat. Nos. 3,250,702 and 3,250,703, assigned to the assignee of the present invention. Ion exchange resins in a size range of about 10 to 250 microns are referred to herein as "finely divided" resin particles, while resin particles in the size range of about 20 to 60 mesh (about 840 to about 250 microns) are referred to as "large bead" resin particles.
A method of producing ion exchange resin particles smaller than 250 microns from large bead resins is known in the art, such as disclosed in U.S. Pat. No. 3,409,566, assigned to the assignee of the present invention. This method comprises grinding the large bead resin particles in a hammer mill to a size range of about 60 to 400 mesh, with a major portion by weight of the resulting resin particles being in a size range of about 200 to 400 mesh (74 to 37 microns).
According to the prior art method, a mixture of cation and anion exchange resins in the size range of about 60 to 400 mesh is prepared in two steps. First, large bead cation and anion exchange resins are ground separately, to produce resin particles predominately in the size range of about 60 to 400 mesh. Then the cation and anion exchange resins are mixed in suitable ratios for use in preparing a slurry of resin particles to be precoated on a filter element. Such a mixing operation is typically performed by an operation of the filtration unit in which the resins are to be used.
Mixing the anion and cation exchange resins after grinding, as in the prior art, produces a combination of resins that includes a number of resin particles of the large bead size and a number of particles of a size smaller than about 32 microns, hereinafter referred to as "fines." It is known in the art that it is desirable to minimize the number of fines and large bead resins in a combination of resin particles.
In the prior art, cation exchange resin fines are often removed and discarded after grinding the cation exchange resin, to reduce the possibility of fouling a filter on which the cation exchange resin will be precoated. Anion exchange resin fines are usually not removed and discarded because anion exchange resins are generally more expensive than cation exchange resins.